What happens if you throw a scalar field into General Relativity? And if you throw more than one? Here is a paper on the phenomenology of neutron stars in theories with more than one scalar field coupled to gravity.

Michael Horbatsch, Hector O. Silva, Davide Gerosa, Paolo Pani, Emanuele Berti, Leonardo Gualtieri, Ulrich Sperhake.
Classical and Quantum Gravity 32 (2015) 20, 204001.
arXiv:1505.07462 [gr-qc].
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