  This paper presents updated estimates of source parameters for GW150914, a
binary black-hole coalescence event detected by the Laser Interferometer
Gravitational-wave Observatory (LIGO) on September 14, 2015 [1]. Reference
presented parameter estimation [2] of the source using a 13-dimensional,
phenomenological precessing-spin model (precessing IMRPhenom) and a
11-dimensional nonprecessing effective-one-body (EOB) model calibrated to
numerical-relativity simulations, which forces spin alignment (nonprecessing
EOBNR). Here we present new results that include a 15-dimensional
precessing-spin waveform model (precessing EOBNR) developed within the EOB
formalism. We find good agreement with the parameters estimated previously [2],
and we quote updated component masses of $35^{+5}_{-3}\mathrm{M}_\odot$ and
$30^{+3}_{-4}\mathrm{M}_\odot$ (where errors correspond to 90% symmetric
credible intervals). We also present slightly tighter constraints on the
dimensionless spin magnitudes of the two black holes, with a primary spin
estimate $0.65$ and a secondary spin estimate $0.75$ at 90% probability.
Reference [2] estimated the systematic parameter-extraction errors due to
waveform-model uncertainty by combining the posterior probability densities of
precessing IMRPhenom and nonprecessing EOBNR. Here we find that the two
precessing-spin models are in closer agreement, suggesting that these
systematic errors are smaller than previously quoted.
