Calculate the Bohr radius of an exciton in nanometers [nm] for a semiconductor with the following carrier effective masses and dielectric constant:

m*_e=0.4

m*_h=0.56

epsilon_r=9.8

a_0=5.28*1e-11 [m]

I know that the bohr radius is a*_b = epsilon_r(mass/mu)a_b where a_b is 0.053. How do I get "m" from the m*_e and m*_h