Towards a new model of the compact triple system łambda Tauri I. Improved periods and preliminary elements of the triple system

We analysed our new radial velocity measurements and photometric observations (covering a time interval of 46838 days and 39100 days, respectively) as well as those from previous studies to derive accurate ephemerides for both orbits of the compact triple system łae. In the case of the inner eclipsing subsystem, we obtained a linear ephemeris of T_ ̊m min.I = ̊m HJD ,2440998.86069(35)+3 = E, improving the accuracy of the previous solutions by more than an order of magnitude. This allowed us to reproduce well all available light curve minima, including the first one recorded in 1844. Furthermore, the period of the outer subsystem, determined from radial velocity analysis, appears to be constant over the interval of available observations, with the corresponding linear ephemeris being T_ ̊m periast. ̊m HJD ,2456822.5(3.6)+33 E,. Assuming near coplanarity of the inner and outer orbits, we estimated the preliminary physical properties of the whole system. We found that the secondary of the inner subsystem is filling or nearly filling its Roche lobe. We used the long-term stability of the eclipsing subsystem period to place a strong upper limit on the rate of the proposed episodic mass transfer to its primary component. Our results constitute a preparatory step for a follow-up study that will also include an analysis of a rich collection of optical spectro-interferometric observations and use a full-fledged numerical model to describe the evolution of the orbital architecture of the system. Particular attention will be focused on determining the inner orbit forced eccentricity and understanding the absence of the apsidal advance of the outer orbit in our present solution.