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Using the Spitzer Space Telescope, we observed a transit at 3.6um of KELT-11b(Pepper et al. 2017). We also observed three partial transits from the ground.We simultaneously fit these observations, ground-based photometry from Pepperet al. (2017), radial velocity data from Pepper et al. (2017), and an SED modelutilizing catalog magnitudes and the Hipparcos parallax to the system. The onlysignificant difference between our results and Pepper et al. (2017) is that wefind the orbital period is shorter by 37 seconds, $4.73610\pm0.00003$ vs.$4.73653\pm0.00006$ days, and we measure a transit center time of BJD_TDB$2457483.4310\pm0.0007$, which is 42 minutes earlier than predicted. Using ournew photometry, we measure the density of the star KELT-11 to 4%. By combiningthe parallax and catalog magnitudes of the system, we are able to measureKELT-11b's radius essentially empirically. Coupled with the stellar density,this gives a parallactic mass and radius of $1.8\,{\rm M}_\odot$ and $2.9\,{\rmR}_\odot$, which are each approximately $1\,\sigma$ higher than the adoptedmodel-estimated mass and radius. If we conduct the same fit using the expectedparallax uncertainty from the final Gaia data release, this differenceincreases to $4\,\sigma$. This demonstrates the role that precise Gaiaparallaxes, coupled with simultaneous photometric, RV, and SED fitting, canplay in determining stellar and planetary parameters. With high precisionphotometry of transiting planets and high precision Gaia parallaxes, theparallactic mass and radius uncertainties of stars become 1% and 3%,respectively. TESS is expected to discover 60 to 80 systems where thesemeasurements will be possible. These parallactic mass and radius measurementshave uncertainties small enough that they may provide observational input intothe stellar models themselves.

Determining Empirical Stellar Masses and Radii Using Transits and Gaia Parallaxes as Illustrated by Spitzer Observations of KELT-11b