Chromospherically active stars. I - HD 136905

The variable star HD 136905, recently designated GX Librae, is a chromospherically active K1 III single-lined spectroscopic binary with a period of 11.1345 days. It has moderate strength Ca II H and K and ultraviolet emission features, while H-alpha is strongly in absorption. The inclination of the system is 58 + or - 17 deg and the unseen secondary is most likely a G or K dwarf. The v sin i of the primary, 32 + or - 2 km/s, results in a minimum radius of 7.0 + or - 0.4 solar radii. Since the star fills a substantial fracture of its Roche lab, the double-peaked limit curve seen by photometric observers is predominantly ellipsoidal in nature. Both the photometry and the spectroscopy yield values for the period and the time of conjunction that are identical within their uncertainties.