**4. Conclusions**

We presented a theoretical analysis of the spin polarization of photoelectrons in the two-color ionization of atoms by short circularly polarized XUV + optical pulses. Based on the non-relativistic SFA model, we showed that spin polarization of the main line and the sidebands can be large. Basically, it is determined by the spin polarization induced by the XUV ionization. Spin polarization in the sidebands depends strongly on the helicity of the pulses and varies with the order of the sideband. Simple approximate expressions are suggested for the spin polarization of the main line and the sidebands. As an example, the spin polarization of the 5p photoelectrons from Xe atoms is calculated. Measurements of the spin polarization in two-color experiments could give important information about the photo processes in the continuum.

**Author Contributions:** All authors have contributed to the manuscript equally. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding

**Acknowledgments:** The authors are grateful to M. Meyer for numerous useful discussions and for critical reading the manuscript. NMK is grateful to DESY (Hamburg) for hospitality and for the financial support and to Donostia International Physics Center (DIPC) for the financial support .

**Conflicts of Interest:** The authors declare no conflict of interests.
