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WR~21a was known as a massive spectroscopic binary composed of an O2.5If*/WN6ha primary and an O3 V((f*))z secondary. Although a minimum value, themass estimated for the primary placed it as one of the most massive stars foundin our Galaxy. We report the discovery of photometric variations in the timeseries observations carried out by the Transiting Exoplanet Survey Satellite(TESS). These light variations are interpreted as formed by two maincomponents: a sharp partial eclipse of the O3 secondary by the O2.5/WN6 star,and tidally excited oscillations. Based on the light minima a new ephemeris forthe system is calculated. The system configuration is detached and the observedeclipse corresponds to the periastron passage. During the eclipse, the lightcurve shape suggests the presence of the heartbeat effect. The frequenciesderived for the tidally excited oscillations are harmonics of the orbitalperiod. Combining new and previously published radial velocity measurements, anew spectroscopic orbital solution is also obtained. Using the \textsc{phoebe}code we model the \textit{TESS} light curve and determine stellar radii of$R_{\rm O2.5/WN6}=23.4$ R$_\odot$ and $R_{\rm O3}=14.3$ R$_\odot$ and anorbital inclination $i=62^\circ\!\!.2\pm0^\circ\!\!.9$. The latter combinedwith the spectroscopic minimum masses lead to absolute masses of $M_{\rmO2.5/WN6}=93.2$ M$_\odot$ and $M_{\rm O3}=52.9$ M$_\odot$, which establishesWR21a as belonging to the rare group of the very massive stars.

The winking eye of a very massive star: WR 21a revealed as an eclipsing binary by TESS