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

EBLM J0608$-$59/TOI-1338/BEBOP-1 is a 12ᵗʰ-magnitude, F9 V star in an eclipsing binary with a much fainter M dwarf companion on a wide, eccentric orbit (P = 14.6 d). The binary is orbited by two circumbinary planets: one transiting on a 95-d orbit and one non-transiting on a 215-d orbit. We have used high-precision photometry from the TESS mission combined with direct mass measurements for the two stars published recently to measure the following model-independent radii: R₁ = 1.32 ± 0.02 R, R₂ = 0.309 ± 0.004 R. Using R₁ and the parallax from Gaia EDR3, we find that this star’s angular diameter is θ = 0.0309 ± 0.0005 mas. The apparent bolometric flux of the primary star corrected for both extinction and the contribution from the M dwarf (< 0.4 per cent) is F_⊕ ,0 = (0.417± 0.005)× 10⁻⁹ erg cm⁻² s⁻¹. Hence, this F9 V star has an effective temperature Teff,1 = 6031 \, K ± 46 \, K\, (rnd.) ± 10 \, K\, (sys.). EBLM J0608$-$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.