BL Herculis

The variability of BL Herculis was discovered by Cuno Hoffmeister, and announced in 1929. Early observations of the star produced a very inaccurate period of 4.2 days, which resulted in peculiar light and radial velocity curves. The first accurate period, 1.3 days, was published by Pavel Parenago in 1940. and a far more precise period of 1.30744185 days was derived from photometric observations in 1983. The descending portion of the star's light curve shows a "bump" (near phase=0.3, with peak brightness phase defined as 0), which models suggest arises from a 2:1 resonance between the fundamental and second overtone pulsation modes. This bump is considered the primary characteristic of BL Her stars, although its position relative to peak brightness varies as a function of the star's period.

The mass of BL Herculis is estimated to be about 0.75 solar masses, just slightly greater than the mass of a typical RR Lyrae variable.