A photometric and astrometric study of the open clusters NGC 1664 and
  NGC 6939
  This study calculated astrophysical parameters, as well as kinematic and
galactic orbital parameters, of the open clusters NGC 1664 and NGC 6939. The
work is based on CCD UBV and Gaia photometric and astrometric data from ground
and space-based observations. Considering Gaia Early Data Release 3 (EDR3)
astrometric data, we determined membership probabilities of stars located in
both of the clusters. We used two-color diagrams to determine $E(B-V)$ color
excesses for NGC 1664 and NGC 6939 as $0.190 \pm 0.018$ and $0.380 \pm 0.025$
mag, respectively. Photometric metallicities for the two clusters were
estimated as [Fe/H] = $-0.10 \pm 0.02$ dex for NGC 1664 and as [Fe/H] = $-0.06
\pm 0.01$ dex for NGC 6939. Using the reddening and metallicity calculated in
the study, we obtained distance moduli and ages of the clusters by fitting
PARSEC isochrones to the color-magnitude diagrams based on the most likely
member stars. Isochrone fitting distances are $1289 \pm 47$ pc and $1716 \pm
87$ pc, which coincide with ages of $675 \pm 50$ Myr and $1.5 \pm 0.2$ Gyr for
NGC 1664 and NGC 6939, respectively. We also derived the distances to the
clusters using Gaia trigonometric parallaxes and compared these estimates with
the literature. We concluded that the results are in good agreement with those
given by the current study. Present day mass function slopes were calculated as
$\Gamma=-1.22\pm0.33$ and $\Gamma=-1.18\pm0.21$ for NGC 1664 and NGC 6939,
respectively, which are compatible with the Salpeter (1955) slope. Analyses
showed that both of clusters are dynamically relaxed. The kinematic and dynamic
orbital parameters of the clusters were calculated, indicating that the
birthplaces of the clusters are outside the solar circle.
