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The detection of gravitational wave signals by Advanced LIGO and AdvancedVirgo enables us to probe the polarization content of gravitational waves. Ingeneral relativity, only tensor modes are present, while in a variety ofalternative theories one can also have vector or scalar modes. Recently testwere performed which compared Bayesian evidences for the hypotheses that eitherpurely tensor, purely vector, or purely scalar polarizations were present.Indeed, with only three detectors in a network and allowing for mixtures oftensor polarizations and alternative polarization states, it is not possible toidentify precisely which non-standard polarizations might be in the signal andby what amounts. However, we demonstrate that one can still infer whether, inaddition to tensor polarizations, alternative polarizations are present in thefirst place, irrespective of the detailed polarization content. We develop twomethods to do this for sources with electromagnetic counterparts, both based onthe so-called null stream. Apart from being able to detect mixtures of tensorand alternative polarizations, these have the added advantage that no waveformmodels are needed, and signals from any kind of transient source with known skyposition can be used. Both formalisms allow us to combine information frommultiple sources so as to arrive at increasingly more stringent bounds. For nowwe apply these on the binary neutron star signal GW170817, showing consistencywith the tensor-only hypothesis with p-values of 0.315 and 0.790 for the twomethods.

Generic searches for alternative gravitational wave polarizations with networks of interferometric detectors