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 \pm 0.002 d , dynamical mass ratio M _ { B } / M _ { A } = 0.911 \pm 0.015 , and spectroscopic radius ratio R _ { B } / R _ { A } = 0.939 \pm 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 % 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 .