Color−magnitude diagram of Mercer 30 up to a radius of 45′′, with dotted lines enclosing the color range used for selection of hot massive cluster members. Black dots are VVV data and blue dots are the HST photometric measurements converted to the VVV passbands. Spectroscopically observed objects are highlighted with symbols, even if they are located at r> 45′′. A, B, and C are the three candidate high-luminosity cluster members that will be discussed in Sect. 3.8.