Determination of R_V for 30 Dor. In the R136 region a): the M_V of single O-type stars derived from HST/WFC3 photometry is plotted against the expected M_V derived from VLT-MAD K-band photometry after applying the Martins & Plez (2006) calibration. In the MEDUSA region b): the M_V derived from VFTS photometry is plotted against the expected M_V derived from IRSF K-band photometry after applying the Martins & Plez (2006) calibration. Solid lines indicate the 1:1 relations.