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We have revisited the sdB+K-star long-period binary PG 1018-047 based on 20new high-resolution VLT/UVES spectra that provided regular coverage over aperiod of more than 26 months. We refine the period and establish that theorbit is significantly eccentric ($P=751.6\pm1.9\rm{d}$ and $e=0.049\pm0.008$).A simultaneous fit derived from the narrow metal lines visible in the spectrumof the sdB star and the metal lines in the red part of the spectrum thatoriginate from the companion provides the mass ratio, $M_{\rm MS}/M_{\rm sdB} =1.52 \pm 0.04$, for the system. From an NLTE model atmosphere analysis of thecombined spectra, we find $T_{\rm eff}$=29900$\pm$330K, log$g$=5.65$\pm$0.06dex and log($n_{\rm He}$/$n_{\rm H}$)=-3.98$\pm$0.16dex for theprimary, consistent with a B-type hot subdwarf star. The spectral contributionof the companion is consistent with a K5V-type star. With the companion havinga mass of only $\sim$0.7$\rm M_{\odot}$ this system lies close to the boundarybelow which stable Roche-lobe overflow (RLOF) cannot be supported. To model theevolution of such a system, we have extended earlier MESA models towards lowercompanion masses. We find that both phase-dependent mass loss during RLOF, when30 to 40\% of the available mass is lost through the outer Lagrange point, andphase-dependent mass loss during RLOF in combination with a circumbinary diskof maximum $M_{\rm CB} = 0.001\rm M_{\odot}$ could have formed the PG1018-047binary system.

Evolutionary Constraints on the Long-period Subdwarf B Binary PG 1018-047