We report on the multi-frequency timing observations of 21 pulsars discovered in the LOFAR Tied-Array All-Sky Survey ( LOTAAS ) .
The timing data were taken at central frequencies of 149 MHz ( LOFAR ) as well as 334 and 1532 MHz ( Lovell Telecope ) .
The sample of pulsars includes 20 isolated pulsars and the first binary pulsar discovered by the survey , PSR J1658 + 3630 .
We modelled the timing properties of the pulsars , which showed that they have , on average , larger characteristic ages .
We present the pulse profiles of the pulsars across the three observing bands , where PSR J1643 + 1338 showed profile evolution that appears not to be well-described by the radius-to-frequency-mapping model .
Furthermore , we modelled the spectra of the pulsars across the same observing bands , using a simple power law , and found an average spectral index of -1.9 \pm 0.5 .
Amongst the pulsars studied here , PSR J1657 + 3304 showed large flux density variations of a factor of 10 over 300 days , as well as mode changing and nulling on timescales of a few minutes .
We modelled the rotational and orbital properties of PSR J1658 + 3630 , which has a spin period of 33 ms in a binary orbit of 3.0 days with a companion of minimum mass of 0.87 M _ { \odot } , likely a Carbon-Oxygen or Oxygen-Neon-Magnesium type white dwarf .
PSR J1658 + 3630 has a dispersion measure of 3.0 pc cm ^ { -3 } , making it possibly one of the closest binary pulsars known .