The eta Aquilae System: Radial Velocities and Astrometry in Search of
  eta Aql B
  The classical Cepheid eta Aql was not included in past Leavitt Law work
(Benedict et al. 2007) because of a presumed complicating orbit due to a known
B9.8V companion. To determine the orbit of eta Aql B, we analyze a significant
number of radial velocity measures (RV) from eight sources. With these we
establish the RV variation due to Cepheid pulsation, using a twelve Fourier
coefficient model, while solving for velocity offsets required to bring the RV
data sets into coincidence. RV residuals provide no evidence of orbital motion,
suggesting either nearly face-on orientation or very long period. Reanalysis of
Hubble Space Telescope Fine Guidance Sensor astrometry now includes reference
star parallax and proper motion priors from Gaia EDR3. As modeling
confirmation, we reanalyze zeta Gem in parallel, deriving zeta Gem parallax and
proper motion values consistent with Gaia EDR3, and consistent with the
Benedict 2007 Leavitt Law. In an effort to further characterize eta Aql B, we
hypothesize that eta Aql residuals larger than those of the associated
reference stars or a parallax inconsistent with EDR3 and the Benedict 2007
Leavitt Law indicate unmodeled orbital motion. Using the astrometric noise or
parallax mismatch with EDR3 we estimate possible periods and mass for eta Aql
B. Ascribing photocenter motion to the photometric variation of the Cepheid,
eta Aql A, yields a plausible separation, consistent with a long period,
explaining the lack of RV variation. None of these approaches yields an
unassailable characterization of the eta Aql A-B system
