Long term optical variability of two HMXB

Abstract: We discuss the optical light curves of two Be X-ray Binaries, IGR J06074+2205 and SAX J2103.5+4545 recovered from the ATLAS, ZTF and ASAS-SN databases. Both sources show long term optical variability of 620 and 420 days respectively, with color redder when brighter. We suggest that this is due to the precession of the circumstellar disk. Another possibility is the propagation of a density wave in the disk. We remark that only these two sources show a large amplitude, periodic optical variability out of a sample of 16 well studied High Mass X-ray Binaries (HMXB) with a Main Sequence primary star.