Toward Understanding the B[e] Phenomenon. IX. Nature and Binarity of MWC645

We present the results of optical and near-IR spectroscopy and multicolor photometry of the emission-line star MWC645, which exhibits the B[e] phenomenon. The presence of positionally variable absorption lines of a cool star detected for the first time indicates that the object is a binary system. Using a combination of the photometric and spectroscopic data as well as the Gaia EDR3 distance (D = 6.5 +/- 0.9 kpc), we disentangled the components' contributions and estimated their surface temperatures and luminosities (18, 000 +/- 2000 K and 4250 +/- 250 K, log L/L-circle dot = 4.0 +/- 0.5 and 3.1 +/- 0.3 for the hot and cool components, respectively). Quasi-cyclic short-period (months) and long-period (similar to 4 yr) photometric variations were detected in both optical and near-IR regions, and are most likely due to variable circumstellar extinction. Our analysis suggests that MWC645 is a member of the FS CMa group. The object's properties can be described by an evolutionary model of a close binary system that currently undergoes a nonconservative mass transfer between intermediate-mass stars (e.g., 7 M-circle dot + 2.8 M-circle dot).