**1. Introduction**

GR 290 (M 33 V0532 = Romano's Star)<sup>1</sup> is a variable star in M 33 galaxy discovered by Giuliano Romano [1] who originally constructed its light curve and classified it as a Hubble-Sandage variable based on its photometric properties. Later, in 1984, Peter Conti [2] introduced a new class of objects which assimilated Hubble-Sandage variables—luminous blue variables (LBV), and thus GR 290 became an LBV candidate [3,4]. This classification has later been supported by the spectroscopic [5] and photometric [6] studies, as well as by its large bolometric luminosity [7]. However, some arguments sugges<sup>t</sup> that the objects is rather on a post-LBV stage already [8–10].

<sup>1</sup> The object has coordinates *α* = 01 : 35 : 09.701, *δ* = +30 : 41 : 57.17 at J2000 epoch.

Romano's star displays both strong spectral and photometric variability, with several significant (about 1.5–2 mag) increases of brightness detected during its long monitoring (Polcaro et al. [10] and references therein). Such variability is typical for LBV stars, while in the Hertzsprung-Russell (H-R) GR 290 lies in Wolf-Rayet (WR) stars region, beyond LBV instability strip [10]. GR 290 is presently in a short, and thus very rare, transition phase between the LBV evolutionary phase and the nitrogen rich WR stellar class (WN). It is an extremely important target for studies of massive star evolution, especially the evolutionary link between LBVs, WR stars and supernovae (SNe).

In this paper we summarise the main results achieved in the study of Romano's star. We combine new studies of GR 290's vicinity (Section 2) with its updated century-long photometric light curve (Section 3). Then, based on spectral data, numerical simulations of its stellar atmosphere (Section 4) and the nebula surrounding it (Section 5), we discuss the current evolutionary stage of the star in Section 6.

### **2. Stellar Vicinity of Romano's Star**

GR 290 is located in the outer spiral arm of the M 33 galaxy, and lies to the east of the OB 88 and OB 89 associations [11–13], located at 0.5 and 0.125 kpc projected distances2, respectively. The most detailed information about photometry of stars in this area may be found in Massey et al. [15]. Figure 1 shows the identification chart of the object and its vicinity, with red symbols corresponding to the stars which were spectrally classified by Massey et al. [16]. Coordinates and spectral classes of the stars are listed in Table 1.

Massey and Johnson [17] found a couple of carbon-rich Wolf-Rayet (WC) stars in these associations, J013458.89+304129.0 (WC4) in OB 88 and J013505.37+304114.9 (WC4-5) in OB 89. Moreover, the OB 88 association contains the star J013500.30+304150.9 classified as an LBV candidate by Massey et al. [18], and later reclassified by Humphreys et al. [9] as a FeII emission-line star. The presence of evolved massive stars in the associations indicates that their age is close to that of GR 290 and that they might have a common origin. Therefore, it is quite reasonable to suppose that GR 290 might have been originally ejected from the OB 89 association. Then, assuming a median escape velocity for runaway stars of 40–200 km/s [19], this ejection would have to have occurred 3.0–0.6 Myr ago, which is consistent with the evolutionary age of GR 290 and with the age of the OB 89 association.

<sup>2</sup> The adopted distance to M 33 is 847 ± 61 kpc (distance module 24.64 ± 0.15) from Galleti et al. [14].


**Table 1.** Stars in the vicinity of GR 290 spectrally classified by Massey et al. [16] (and references therein). Names and coordinates are given according to Massey et al. [15]. The three stars also included in Table 2 are marked by boldface.

*a* later classified as Of/late-WN by Humphreys et al. [20]; *b* later classified as a FeII emission-line star by Humphreys et al. [9].

**Figure 1.** Identification chart of GR 290 vicinity and OB 88 and OB 89 associations. The colour picture is a combination of three direct images, with blue corresponding to B filter, green—to V and red—to R filter, all obtained with 2.5 m telescope of the Caucasian Mountain Observatory (CMO) of the Sternberg Astronomical Institute of Moscow State University. Green circles mark the stars studied in this work and red ones studied by Massey et al. [15,16]. Red squares are stars considered to be foreground objects by Massey et al. [15].

The field around GR 290 is not ye<sup>t</sup> sufficiently explored as it consists mostly of faint (*V* > 18 mag) stars that require large telescopes for acquiring the spectra. Fortunately, some of the surrounding stars happened to lay on the slit during the long-slit observations of the object, thus that analysis of such data may provide additional information on the stellar contents and interstellar extinction in the vicinity of Romano's star. Therefore, we retrieved from General observational archive of Special Astrophysical observatory of Russian Academy of Sciences (SAO RAS)<sup>3</sup> all long-slit spectra of GR 290 obtained on Russian 6-m telescope with the Spectral Camera with Optical Reducer for Photometric and Interferometric Observations (SCORPIO) [21] during the years 2005–2016. We also utilised the spectra obtained with the OSIRIS spectrograph on the *Gran Telescopio Canarias (GTC)* and analysed by Maryeva et al. [22] and Maryeva et al. [23]. We reduced these spectra in a uniform way using the ScoRe package<sup>4</sup> initially created for the SCORPIO data reduction, and extracted the spectra of all stars crossed by the slit. To perform the spectral classification of these stars, we used an automatic code based on the *χ*2 fitting with spectral standards from STELIB<sup>5</sup> (see Le Borgne et al. [24]) in the same way as used by Maryeva et al. [25]. The stars with spectra extracted and analysed in this way are

<sup>3</sup> General observational archive of Special Astrophysical observatory is available at https://www.sao.ru/oasis/cgi-bin/fetch? lang=en.

<sup>4</sup> ScoRE package available at http://www.sao.ru/hq/ssl/maryeva/score.htm.

<sup>5</sup> STELIB is availabte at http://webast.ast.obs-mip.fr/stelib.

marked with green circles in Figure 1, and their estimated spectral classes, measured positions and photometric magnitudes are listed in Table 2. The resulting spectra of the stars in flux units are shown in Figures A1–A5.


**Table 2.** The sample of stars in the field around GR 290 studied in this work. N corresponds to the labels in Figure 1. *V* and (*B* − *V*) taken from Massey et al. [15].

*a*,*e* Stars classified as RSG by Massey et al. [16]; *b* star classified as O6III(f)+Neb by Massey et al. [16]; *<sup>c</sup>*,*d* stars classified as foreground objects by Massey et al. [15].

As we can see in Figure 1, our sample of stars partially intersect with the ones studied by Massey et al. [15,16]. We were able to refine the estimates of spectral classes for J013459.81+304156.9 and J013507.43+304132.6, classified earlier as just red supergiants (RSG) [16], as well as for J013506.17+304129.1 and J013507.18+304156.4 as foreground objects according to Massey *et al.* [15]. Our sample contains three more RSGs, which were not previously reported, and four hot stars, with only one (J013505.74+304101.9 with O6III(f)+Neb spectral class) known before [16]. Among three others, the spectrum of J013505.76+304102.21 displays the He I emission and strong nebular lines. The second one, J013514.1+304423.21, has a spectral slope corresponding to high temperature, and shows H and He absorption lines, while the last, J013501.87+304157.3, was preliminary classified as B5–B7 supergiant.

Knowing the spectral classes of these stars, and therefore their intrinsic colour indices, allows us to estimate the interstellar extinction around GR 290. Its value is comparable to the galactic foreground extinction value of *<sup>E</sup>*(*<sup>B</sup>*−*<sup>V</sup>*) = 0.052 (according to the NED extinction calculator [26]). We did not register any star with higher reddening in the vicinity of GR 290.
