On Cepheid Distance Scale Bias due to Stellar Companions and Cluster
  Populations
  State-of-the art photometric measurements of extragalactic Cepheids account
for the mean additional light due to chance superposition of Cepheids on
crowded backgrounds through the use of artificial star measurements. However,
light from stars physically associated with Cepheids may bias relative distance
measurements if the changing spatial resolution along the distance ladder
significantly alters the amount of associated blending. We have identified two
regimes where this phenomenon may occur: Cepheids in wide binaries and open
clusters.
  We estimate stellar association bias using the photometric passbands and
reddening-free Wesenheit magnitudes used to set up the distance scale. For wide
binaries, we rely on Geneva stellar evolution models in conjunction with
detailed statistics on intermediate-mass binary stars. For the impact of
cluster stars, we have compiled information on the clustered Cepheid fraction
and measured the typical cluster contribution in M31 via deep HST imaging
provided by the PHAT project.
  We find that the dominant effect on the distance scale comes from Cepheids in
clusters, despite cluster Cepheids being a relatively rare phenomenon. Wide
binaries have a negligible effect of 0.004% on $H_0$ for long-period Cepheids
observed in the near-infrared or when considering Wesenheit magnitudes. We
estimate that blending due to cluster populations has previously resulted in a
0.23% overestimate of $H_0$. Correcting for this bias, we obtain $H_0 = 73.07
\pm 1.76 km s^{-1} Mpc^{-1}$, which remains in $3.3\sigma$ tension with the
Planck value. We conclude that stellar association bias does not constitute a
limit for measuring $H_0$ with an accuracy of 1%.
