The confirmation of the existence of black holes is one of the most basic results in astrophysics. There is a wide range of masses of black holes, from those with stellar mass, which are the result of the catastrophic final phase of very massive stars, to the supermassive black holes at the centres of most galaxies. The mass of a black hole is up to now the only parameter which scientists are able to measure. In this work, we present an original method for measuring the masses of black holes, from those of stellar mass to the supermassive variety, based on a simple measurement of the

We present 12 epochs of optical spectroscopy taken across the discovery outburst of the black hole (BH) candidate MAXI J1803-298 with the Gran Telescopio Canarias and Very Large Telescope. The source followed a standard outburst evolution. This means it passed through the so-called "hard" and "soft" states, defined in terms of the relative contribution of high to low energy X-rays. The system displays a "triangular" shape in the hardness intensity diagram, consistent with that seen in high-inclination BH transients and the previously reported detection of X-ray dips. The two epochs observed

2MASS J20395358+4222505 is an obscured early B supergiant near the massive OB star association Cyg OB2. Despite its bright infrared magnitude (Ks= 5.82) it has remained largely ignored because of its dim optical magnitude (B= 16.63, V= 13.68). In a previous work we classified it as a highly reddened, potentially extremely luminous, early B-type supergiant. We obtained its spectrum in the U, B and R spectral bands during commissioning observations with the instrument MEGARA@GTC. It displays a particularly strong Hα emission for its spectral type, B1 Ia. The star seems to be in an intermediate