  Long-range interactions between cold Rydberg atoms, which are used in many
important applications, can be enhanced using F\"orster resonances between
collective many-body states controlled by an external electric field. Here we
report on the first experimental observation of highly-resolved
radio-frequency-assisted F\"orster resonances in a few cold Rb Rydberg atoms.
We also observed radio-frequency-induced F\"orster resonances which cannot be
tuned by a dc electric field. They imply an efficient transition from van der
Waals to resonant dipole-dipole interaction due to Floquet sidebands of Rydberg
levels appearing in the rf-field. This method can be applied to enhance the
interactions of almost arbitrary Rydberg atoms with large principal quantum
numbers.
