Context : Reports on a transient source about 1.25 \degr south of the Galactic Centre motivated these follow-up observations with the WSRT and the reinvestigation of archival VLA data .
The source GCRT J1745-3009 was detected during a 2002 Galactic Centre monitoring programme with the VLA at 92 cm by five powerful 10-min bursts with a 77-min recurrence while apparently lacking any interburst emission .

Aims : The WSRT observations were performed and archival VLA data were reduced to re-detect GCRT J1745-3009 at different epochs and frequencies , to constrain its distance and determine its nature .
We attempted to extract a more accurate lightcurve from the discovery dataset of GCRT J1745-3009 in order to rule out some of the models that have been suggested .
We also investigated transient behaviour of a nearby source .

Methods : The WSRT data were taken in the ” maxi-short ” configuration , using 10 s integrations , on 2005 March 24 at 92 cm and on 2005 May 14/15 at 21 cm .
Five of the six VLA observations we reduced are the oldest of this field in this band .

Results : GCRT J1745-3009 was not redetected .
With the WSRT we reached an rms sensitivity of 0.21 mJy \mathrm { beam ^ { -1 } } at 21 cm and 3.7 mJy \mathrm { beam ^ { -1 } } at 92 cm .
Reanalysis of the discovery observation data resulted in a more accurate and more complete lightcurve .
The five bursts appear to have the same shape : a steep rise , a more gradual brightening and a steep decay .
We found variations in burst duration of order \simeq 3 \% .
We improved the accuracy of the recurrence period of the bursts by an order of magnitude : 77.012 \pm 0.021 min .
We found no evidence of aperiodicity .
We derived a very steep spectral index : \alpha = -6.5 \pm 3.4 .
We improved the 5 \sigma upper limits for interburst emission and fractional circular polarization to 31 mJy \mathrm { beam ^ { -1 } } and 8 \% , respectively .
Transient behaviour of a nearby source could not be established .

Conclusions : Models that predict symmetric bursts can be ruled out , but rotating systems are favoured , because their periodicity is precise .
Scattering constraints imply that GCRT J1745-3009 can not be located far beyond the GC .
If this source is an incoherent emitter and not moving at a relativistic velocity , it must be closer than 14 pc .