def gr2R(g, r):
    """Use the Lupton 2005 transformations listed at
    http://classic.sdss.org/dr7/algorithms/sdssUBVRITransform.html
    to transform g,r passbands into R.

    Returns array of R and float estimate of systematic RMS error of transformation.
    """
    R = r - 0.1837*(g - r) - 0.0971
    sigma = 0.0106
    return R, sigma