2014MNRAS.440.2676S

PG 1544+488 is an exceptional short-period spectroscopic binary containing two subdwarf B stars. It is also exceptional because the surfaces of both components are extremely helium-rich. We present a new analysis of spectroscopy of PG 1544+488 obtained with the William Herschel Telescope. We obtain improved orbital parameters and atmospheric parameters for each component. The orbital period P = 0.496±0.002d, dynamical mass ratio MB/MA = 0.911±0.015 and spectroscopic radius ratio RB/RA = 0.939±0.004 indicate a binary consisting of nearly identical twins. The data are insufficient to distinguish any difference in surface composition between the components, which are slightly metal-poor (1/3 solar) and carbon-rich (0.3 per cent by number). The latter indicates that the hotter component, at least, has ignited helium. The best theoretical model for the origin of PG 1544+488 is by the ejection of a common envelope from a binary system in which both components are giants with helium cores of nearly equal mass. Since precise tuning is necessary to yield two helium cores of similar masses at the same epoch, the mass ratio places very tight constraints on the dimensions of the progenitor system and on the physics of the common-envelope ejection mechanism.