#
# Model
#
def rvmodel(x,ampl,offset,phase):
    return offset+ampl*np.cos(2*np.pi*x+phase)

#
# Parameters: initial values, and bounds
#
par0   = np.array([50., 0., 0.])
lobnd  = np.array([10., -50., 0.])
upbnd  = np.array([100., 50., 2*np.pi])
bounds = (lobnd,upbnd)

#
# Now perform least-squares fit
#
q = optimization.curve_fit(rvmodel, x, y, par0, yerr, bounds=bounds)
ampl_lsq, offset_lsq, phase_lsq = q[0]