  We present ULTRACAM multiband optical photometry of two transitional
millisecond pulsars, PSR J1023+0038 and PSR J1227$-$4853, taken while both were
in their radio pulsar states. The light curves show significant asymmetry about
the flux maxima in all observed bands, suggesting an asymmetric source of
heating in the system. We model the light curves using the Icarus binary code,
using models with an additional "hot spot" heating contribution and an
anisotropic heat redistribution model to treat the asymmetry. Our modelling
reveals companion stars with under-filled Roche lobes in both PSRs J1023+0038
and J1227$-$4853, with Roche lobe filling factors in the range $f \sim
0.82-0.92$. While the volume-averaged filling factors are closer to unity,
significant under-filling is unexpected from tMSPs as they must rapidly
over-fill their Roche lobes to start transferring mass, which occurs on
timescale of weeks or months. We discuss the motivation and validity of our
extensions to the models and the implications of the under-filled Roche lobe,
and suggest future work to further investigate the role of the filling factor
in the tMSP cycle.
