FM CMa: Hot and Massive Eclipsing Binary with a Pulsating Component

Long -term spectroscopic monitoring of the eclipsing system FM CMa obtained between 2017 - 2023 at the Skalnate ' Pleso (SPO, Slovakia) and Cerro-Tololo Interamerican observatories (CTIO, Chile) is presented. Preliminary analysis of the TESS high -precision satellite photometry (sectors 7 and 33, only FFI - Full Frame Image) shows that the eclipses in the system are total, and there is a slow apsidal motion. The light-curve solution requires a substantial amount of third light. In addition, the TESS light curve exhibits low-amplitude variability (in the out-of-eclipse parts), probably caused by radial pulsations with a frequency of about 5.5 cycles/day (4.3 hours). Line profiles show strong asymmetries corresponding to the photometric ephemeris but the possible third component is not visible. The nature of the system is discussed.