Discovery of the optical counterpart to the X-ray pulsar SAX J2103.5+4545

We report optical and infrared photometric and spectroscopic observations that identify the counterpart to the 358.6-s X-ray transient pulsar SAX J2103.5+4545 with a moderately reddened V=14.2 B0Ve star. This identification makes SAX J2103.5+4545 the Be/X-ray binary with the shortest orbital period known, Porb= 12.7 days. The amount of absorption to the system has been estimated to be Av=4.2+-0.3, which for such an early-type star implies a distance of about 6.5 kpc. The optical spectra reveal major and rapid changes in the strength and shape of the Halpha line. The Halpha line was initially observed as a double peak profile with the ratio of the intensities of the blue over the red peak gr…