The triple system DM Persei

Radial velocities derived from 40 medium-dispersion spectrograms acquired between 1970 and 1981, and light curves based on 440 UBV photometric observations, are presented for the truly Algol-type system DM Persei. The spectroscopic data yield a mass ratio of 0.32±0.02 for the binary, and show that it is part of a triple system with an orbital period of ∼100 day and semi-major axis ∼0.9 AU. The third component has a mass (for co-planar orbits) of ∼3.6 M⊙, and since it is not readily detected in the observed spectrum, could possibly be a binary system itself. An analysis of the three light curves by means of the synthesis code LIGHT confirms that the binary system is semi-detached and consists of a B5V primary and an A5III secondary. The parameters of the primary component concur with the empirical mass–luminosity relationship for detached systems. DM Per joins the small group of short-period early-type semi-detached massive systems, which include V Pup and u Her, whose characteristics differ significantly from the classical Algols.