  Observing planetary auroral radio emission is the most promising method to
detect exoplanetary magnetic fields, the knowledge of which will provide
valuable insights into the planet's interior structure, atmospheric escape, and
habitability. We present LOFAR-LBA circularly polarized beamformed observations
of the exoplanetary systems 55 Cancri, $\upsilon$ Andromedae, and $\tau$
Bo\"{o}tis. We tentatively detect circularly polarized bursty emission from the
$\tau$ Bo\"{o}tis system in the range 14-21 MHz with a flux density of
$\sim$890 mJy and with a significance of $\sim$3$\sigma$. For this detection,
no signal is seen in the OFF-beams, and we do not find any potential causes
which might cause false positives. We also tentatively detect slowly variable
circularly polarized emission from $\tau$ Bo\"{o}tis in the range 21-30 MHz
with a flux density of $\sim$400 mJy and with a statistical significance of
$>$8$\sigma$. The slow emission is structured in the time-frequency plane and
shows an excess in the ON-beam with respect to the two simultaneous OFF-beams.
Close examination casts some doubts on the reality of the slowly varying
signal. We discuss in detail all the arguments for and against an actual
detection. Furthermore, a $\sim$2$\sigma$ marginal signal is found from the
$\upsilon$ Andromedae system and no signal is detected from the 55 Cancri
system. Assuming the detected signals are real, we discuss their potential
origin. Their source probably is the $\tau$ Bootis planetary system, and a
possible explanation is radio emission from the exoplanet $\tau$ Bootis b via
the cyclotron maser mechanism. Assuming a planetary origin, we derived limits
for the planetary polar surface magnetic field strength, finding values
compatible with theoretical predictions. Further low-frequency observations are
required to confirm this possible first detection of an exoplanetary radio
signal. [Abridged]
