def spin_in_roche(s, etheta, elongan, eincl):
  """
    Transform the spin s of a star on Kerpler orbit with

      etheta  - true anomaly
      elongan - longitude of ascending node
      eincl - inclination

    from in the plane of sky reference frame into
    the Roche reference frame.
  """
  #  m = Rz(long).Rx(-incl).Rz(theta).Rz(pi)
  m = euler_trans_matrix(etheta, elongan, eincl)

  return np.dot(m.T, s)