We present intermediate resolution spectroscopy of the optical counterpart to the black hole X-ray transient MAXI J1820+070 ( =ASASSN-18ey ) obtained with the OSIRIS spectrograph on the 10.4-m Gran Telescopio Canarias .
The observations were performed with the source close to the quiescent state and before the onset of renewed activity in August 2019 .
We make use of these data and K-type dwarf templates taken with the same instrumental configuration to measure the projected rotational velocity of the donor star .
We find v _ { rot } \sin i = 84 \pm 5 km s ^ { -1 } ( 1 - \sigma ) , which implies a donor to black-hole mass ratio q = { M _ { 2 } } / { M _ { 1 } } = 0.07 \pm 0.01 for the case of a tidally locked and Roche-lobe filling donor star .
The derived dynamical masses for the stellar components are M _ { 1 } = ( 5.89 \pm 0.20 ) \sin ^ { -3 } i M _ { \sun } and M _ { 2 } = ( 0.41 \pm 0.07 ) \sin ^ { -3 } i M _ { \sun } .
The use of q , combined with estimates of the accretion disk size at the time of the optical spectroscopy , allows us to revise our previous orbital inclination constraints to 67 ^ { \circ } < i < 81 ^ { \circ } .
These values lead to 95 % confidence level limits on the masses of 5.71 < M _ { 1 } ( M _ { \odot } ) < 8.10 and 0.28 < M _ { 2 } ( M _ { \odot } ) < 0.71 .
Adopting instead the 63 \pm 3 ^ { \circ } orientation angle of the radio jet as the binary inclination leads to 1 - \sigma values of M _ { 1 } = 8.40 ^ { +0.77 } _ { -0.70 } M _ { \odot } and M _ { 2 } = 0.59 ^ { +0.11 } _ { -0.10 } M _ { \odot } .