  The semi-regular variable star RU Vulpeculae (RU Vul) is being observed
visually since 1935. Its pulsation period and amplitude are declining since
$\sim1954$. A leading hypothesis to explain the period decrease in asymptotic
giant branch (AGB) stars such as RU Vul is an ongoing flash of the He-burning
shell, also called a thermal pulse (TP), inside the star. In this paper, we
present a CCD photometric light curve of RU Vul, derive its fundamental
parameters, and test if the TP hypothesis can describe the observed period
decline. We use CCD photometry to determine the present-day pulsation period
and amplitude in three photometric bands, and high-resolution optical
spectroscopy to derive the fundamental parameters. The period evolution of RU
Vul is compared to predictions by evolutionary models of the AGB phase. We find
that RU Vul is a metal-poor star with a metallicity $[{\rm M}/{\rm
H}]=-1.59\pm0.05$ and an effective surface temperature of $T_{\rm
eff}=3634\pm20$ K. The low metallicity of RU Vul and its kinematics indicate
that it is an old, low-mass member of the thick disc or the halo population.
The present day pulsation period determined from our photometry is $\sim108$ d,
the semi-amplitude in the V-band is $0.39\pm0.03$ mag. The observed period
decline is found to be well matched by an evolutionary AGB model with stellar
parameters comparable to those of RU Vul. We conclude that the TP hypothesis is
in good agreement with the observed period evolution of RU Vul.
