def period(A,M1,M2):
A *= rsun_cm<EOL>print(A)<EOL>velocity = np.sqrt(grav_const*msun_g*(M1+M2)/A)<EOL>print(old_div(velocity,<NUM_LIT>))<EOL>p = <NUM_LIT>*np.pi * A / velocity<EOL>p  /= (<NUM_LIT>*<NUM_LIT>*<NUM_LIT>)<EOL>return p<EOL>
calculate binary period from separation. 

Parameters
----------
A : float
    separation A Rsun.
M1, M2 : float
    M in Msun.

Returns
-------
p
    period in days.