Pre-discovery transits of the exoplanets WASP-18b and WASP-33b from Hipparcos

Abstract

We recover transits of WASP-18b and WASP-33b from Hipparcos (1989-1993) photometry. Marginal detections of HAT-P-56b and HAT-P-2b may be also present in the data. New ephemerides are fitted to WASP-18b and WASP-33b. A tentative (∼1.3σ) orbital decay is measured for WASP-18b, but the implied tidal quality factor (Q^΄ ∼ 5 × 10⁵) is small and survival time (<10⁶ yr) is too short to be likely. No orbital decay is measured for WASP-33b, and a limit of Q^΄ > 2 × 10⁵ is placed. For both planets, the uncertainties in published ephemerides appear underestimated: the uncertainty in the period derivative of WASP-18b would be greatly reduced if its current ephemeris could be better determined.