Fundamental effective temperature measurements for eclipsing binary stars -- V. The circumbinary planet system EBLM J0608-59

EBLM J0608-59 / TOI-1338 / BEBOP-1 is a 12th-magnitude, F9V star in an eclipsing binary with a much fainter M-dwarf companion on a wide, eccentric orbit (P=14.6 d). The binary is orbited by two circumbinary planets: one transiting on a 95-day orbit and one non-transiting on a 215-day orbit. We have used high-precision photometry from the TESS mission combined with direct mass measurements for the two stars published recently to measure the following model-independent radii: $R_1=1.32\pm 0.02R_{\odot }$, $R_2=0.309\pm 0.004R_{\odot }$. Using $R_1$ and the parallax from Gaia EDR3 we find that this star’s angular diameter is $\theta =0.0309\pm 0.0005$ mas. The apparent bolometric flux of the primary star corrected for both extinction and the contribution from the M-dwarf ($<0.4$%) is ${\mathcal{F}}_{\oplus ,0}=(0.417\pm 0.005)\times {10}^{-9}{\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{c}\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. Hence, this F9V star has an effective temperature $T_{\mathrm{e}\mathrm{f}\mathrm{f},1}=6031\mathrm{K}\pm 46\mathrm{K}(\mathrm{r}\mathrm{n}\mathrm{d}\mathrm{.})\pm 10\mathrm{K}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{.})$. EBLM J0608-59 is an ideal benchmark star that can be added to the sample of such systems we are establishing for “end-to-end” tests of the stellar parameters measured by large-scale spectroscopic surveys.