About 50 % of all known hot subdwarf B stars ( sdBs ) reside in close ( short period ) binaries , for which common envelope ejection is the most likely formation mechanism .
However , Han et al . ( 21 ) predict that the majority of sdBs should form through stable mass transfer leading to long period binaries .
Determining orbital periods for these systems is challenging and while the orbital periods of \sim 100 short period systems have been measured , there are no periods measured above 30 days .
As part of a large program to characterise the orbital periods of subdwarf B binaries and their formation history , we have found that PG 1018–047 has an orbital period of 760 \pm 6 days , easily making it the longest period ever detected for a subdwarf B binary .
Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra , we derive the radial velocity amplitudes of both stars , and estimate the mass ratio M _ { MS } / M _ { sdB } = 1.6 \pm 0.2 .
From the combination of visual and infrared photometry , the spectral type of the companion star is determined to be mid K .