Fiducial stellar spectrum of HD 63433

Using Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph, we measure a transit depth of 11.1{+/-}1.5% in the blue wing and 8{+/-}3% in the red. This signal is unlikely to be due to stellar variability, but should be confirmed by an upcoming second transit observation with HST.

We do not detect Ly{alpha} absorption from the inner planet, a smaller R=2.15R{Earth} mini-Neptune on a 7.1day orbit. We use Keck/NIRSPEC to place an upper limit of 0.5% on helium absorption for both planets. We measure the host star's X-ray spectrum and mid-ultraviolet flux with XMM-Newton, and model the outflow from both planets using a 3D hydrodynamic code.