  Precision spectroscopy of atoms and molecules allows one to search for and to
put stringent limits on the variation of fundamental constants. These
experiments are typically interpreted in terms of variations of the fine
structure constant $\alpha$ and the electron to proton mass ratio
$\mu=m_e/m_p$. Atomic spectroscopy is usually less sensitive to other
fundamental constants, unless the hyperfine structure of atomic levels is
studied. However, the number of possible dimensionless constants increases when
we allow for fast variations of the constants, where "fast" is determined by
the time scale of the response of the studied species or experimental apparatus
used. In this case, the relevant dimensionless quantity is, for example, the
ratio $m_e/\langle m_e \rangle$ and $\langle m_e \rangle$ is the time average.
In this sense, one may say that the experimental signal depends on the
variation of dimensionful constants ($m_e$ in this example).
