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Target RA (J2000): 61.331638 and deg Target Dec (J2000): 20.15709 and deg Gaia GMag: 10.08 Programme ID: CH_PR230032 PI of observing programme: Damasso Title of programme: Pinning down orbital period transit ephemeris and radius of the infant planet V1298 Tau e Abstract: V1298 Tau is a very young (20±10 Myr) star that hosts four planets discovered by Kepler/K2. It is a unique laboratory to test planet formation and evolution theories right after system's birth. The outermost planet V1298 Tau e was observed in transit only once by K2 and a second transit was detected later on by TESS thus orbital period and ephemeris are presently not yet constrained. This greatly affects the analysis of radial velocities and the mass determination of the planet. We are intensively studying and monitoring V1298 Tau. Since 2019 we are intensively following it up with HARPS-N and CARMENES to determine planet masses and bulk densities with focus on planet e. So far our published results provided preliminary estimates which need to be revised once the transit ephemeris of planet e will be accurately known. K2 and TESS observations also revealed that the transit depth is hampered by stellar activity thus observing transits with CHEOPS will allow for a more accurate measurement needed to pin down the bulk density of the planet. CHEOPS will provide crucial results to characterise physical and architectural properties of the whole planetary system. CHEOPS observations will complement our large dataset of radial velocities allowing for fully exploitation. The results will be crucial to carry on a thorough study of the formation and dynamical evolution of the system at the earliest phases and to predict the future evolution of planets' atmospheric structure. [truncated! Please see actual data for full text]