  HD149404 is an evolved non-eclipsing O-star binary that has previously
undergone a Roche lobe overflow interaction. Understanding some key properties
of the system requires a determination of the orbital inclination and of the
dimensions of the components. The BRITE-Heweliusz satellite was used to collect
photometric data of HD149404. Additional photometry was retrieved from the SMEI
archive. These data were analysed using a suite of period search tools. The
orbital part of the lightcurve was modelled with the nightfall binary star
code. The Gaia-DR2 parallax of HD149404 was used to provide additional
constraints. The periodograms reveal a clear orbital modulation of the
lightcurve with a peak-to-peak amplitude near 0.04 mag. The remaining
non-orbital part of the variability is consistent with red noise. The
lightcurve folded with the orbital period reveals ellipsoidal variations, but
no eclipses. The minimum when the secondary star is in inferior conjunction is
deeper than the other minimum due to mutual reflection effects between the
stars. Combined with the Gaia-DR2 parallaxes, the photometric data indicate an
orbital inclination in the range of 23{\deg} to 31{\deg} and a Roche lobe
filling factor of the secondary larger than or equal to 0.96. The luminosity of
the primary star is consistent with its present-day mass, whereas the more
evolved secondary appears overluminous for its mass. We confirm that the
primary's rotation period is about half the orbital period. Both features most
probably stem from the past Roche lobe overflow episode.
