Benjamin Montesinos LAEFF and CSIC I have three research interests: 1) Stars with protoplanetary disks. We are studying young PMS stars (1-20 Myr), T Tauri, Vega and Herbig Ae/Be with protoplanetary disks in order to disentangle some of the problems of the evolution of this kind of objects. We are carrying out a detailed study of the stellar properties -effective temperatures, gravities, metallicities, ages, etc- because these are crucial inputs to the self-consistent disk models that we use to reproduce the complete spectral energy distribution of each object. This study is being done in collaboration with Carlos Eiroa (UAM, Madrid), Bruno Merin (Leiden Observatory) and Alcione Mora (IAA, Granada). 2) Activity cycles in late-type stars. In analogy with our Sun, we know that late-type main-sequence stars show magnetic cycles whose lengths span from 2 to 15 years. Rosario Lorente (ESAC), who is finishing now her PhD Thesis on the subject, and I, are applying so-called "interface" dynamo models to predict, on theoretical bases, the length of the cycles and the potential behaviour of the butterfly diagrams for a sample of 25 late-type G and K stars with well-detected cycles. 3) Hydrodynamics of gas in the penumbra of sunspots. We are trying to model the time-dependent behaviour of gas flows in the filaments of the penumbra of sunspots. Since the beginning of the 20th century it was known the existence of gas flows travelling from the inner part of the penumbra towards the outer parts. The flows dissapear abruptly when reaching the outer edge of the spot. We found an explanation of this phenomenon, known as the Evershed effect, in terms of flows along thin magnetic flux tubes. The model, so far, does not contain any time dependence. Given the transient nature of the flow, a more refined treatment is required. This research is done in collaboration with John H. Thomas (University of Rochester, NY) and Luis Sarro (UNED, Madrid).