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We present ULTRACAM multiband optical photometry of two transitionalmillisecond pulsars, PSR J1023+0038 and PSR J1227$-$4853, taken while both werein their radio pulsar states. The light curves show significant asymmetry aboutthe flux maxima in all observed bands, suggesting an asymmetric source ofheating in the system. We model the light curves using the Icarus binary code,using models with an additional "hot spot" heating contribution and ananisotropic heat redistribution model to treat the asymmetry. Our modellingreveals companion stars with under-filled Roche lobes in both PSRs J1023+0038and J1227$-$4853, with Roche lobe filling factors in the range $f \sim0.82-0.92$. While the volume-averaged filling factors are closer to unity,significant under-filling is unexpected from tMSPs as they must rapidlyover-fill their Roche lobes to start transferring mass, which occurs ontimescale of weeks or months. We discuss the motivation and validity of ourextensions to the models and the implications of the under-filled Roche lobe,and suggest future work to further investigate the role of the filling factorin the tMSP cycle.

Optical photometry of two transitional millisecond pulsars in the radio pulsar state