ethraid: A simple method for characterizing long-period companions using Doppler, astrometric, and imaging constraints

We present , an open source Python package designed to measure the mass (m_c) and separation (a) of a bound companion from measurements covering a fraction of the orbital period. constrains m_c and a by jointly modeling radial velocity (RV), astrometric, and/or direct imaging data in a Bayesian framework. Partial orbit data sets, especially those with highly limited phase coverage, are well-represented by a few method-specific summary statistics. By modeling these statistics rather than the original data, optimizes computational efficiency with minimal reduction in accuracy. uses importance sampling to efficiently explore the often broad posteriors that arise from partial orbits. The core computations of are implemented in Cython for speed. We validate 's performance by using it to constrain the masses and separations of the planetary companions to HD 117207 and TOI-1694. We designed to be both fast and simple, and to give broad, "quick look" constraints on companion parameters using minimal data. is pip installable and available on Github.