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tokens: the observed light curves, RVs, and SED of Mon-735. Jointly modelling the SED allows us to propagate our optical spectroscopic light ratio constraints into the IR Spitzer light-curve bands and use them to help constrain the fundamental parameters of both stars. This is particularly important for Mon-735, as the system displays near-equal eclipse depths in the Spitzer light curves, which can lead to a degeneracy between the radius ratio, surface brightness ratio, and inclination when modelling the data. Details of the modelling are given in Sections 3.1 – 3.3 below. 3.1 Light curves The Spitzer 3.6 and 4.5 μ m light curves are shown in Fig. 2 . In addition to eclipses on both the primary and secondary stars, both light curves display reasonably rough aperiodic out-of-eclipse (OOE) variations, which are similar to Spitzer light curves of other young EBs (e.g. Morales-Calderón et al. 2012 ; Gillen et al. 2017a ). Given the rough OOE variations, we opted to use a Matérn kernel with smoothness parameter ν = 3/2 (as approximated within the celerite package), which is appropriate for such rough variations (Rasmussen Williams 2006 ). The Matérn 3/2 covariance kernel k is given by where τ = | t i − t j | is the time interval between times t i and t j , and A and l are the characteristic amplitude and time-scale of the variations, respectively. Figure 2. Spitzer /IRAC 3.6 and 4.5 μ m relative flux light curves of Mon-735 (black points, top and bottom, respectively) with the gp-ebop model in red and residuals below. In both light curves, the red line and pink shaded regions show the mean and 1σ and 2σ confidence intervals of the predictive posterior distribution. The system displays near-equal eclipses on both stars, which are ∼36–37 per cent deep, as well as relatively rough short time-scale out-of-eclipse variability. We applied theoretical constraints on the quadratic limb darkening parameters using the predictions of Claret, Hauschildt Witte ( 2012 ). We used the Claret tables rather than ldtk because the specific intensities of the PHOENIX model spectra (Husser et al. 2013 ), which ldtk is based on, only extend up to a wavelength of 2.6 μ m, so the IRAC 3.6 and 4.5 μ m bandpasses are not covered. We assumed T eff = 3200 K and log g = 4.0 for both the primary and secondary stars (setting Z = 0.0, which is the closest value available to the cluster metallicity; King et al. 2000 ). 2 The T eff and log g values were set based on preliminary fits to the SED data (using PHOENIX models) and the light curves and RVs, respectively, and then rounded to the nearest value in the Claret tables. Uncertainties on the limb darkening coefficients (LDCs) were set based on the spread of coefficient values within a representative range in T eff and log g , which were determined from initial modelling of the light curves and RVs, and further inflated by comparing the Claret LDC values using both the least-square and flux conservation methods. In practice, given the