Observations of fast varying phenomena are often hampered by large "dead" times between the exposures, necessary for technological reasons. However, even the existing infrared detectors are able to deliver duty cycles >= 99%. Here we describe how high-cadence transit timing allows to probe for the presence of additional bodies in the system of known transiting planets. We report transit timing observations of two extrasolar planets. We also review the available instrumentation at the European Southern Observatory that is capable of such observations.