**Table 1:** The values of the stellar mass (in $M_{\odot }$ ) from spectroscopic and asteroseismic studies, and the values obtained in this work from three different methods.
Source	PG 1159-035	PG 2131+066	PG 1707+427	PG 0122+200
Spectroscopy	$0.54 \pm 0.1^{(a)}$	$0.55 \pm 0.1^{(a)}$	$0.54 \pm 0.1^{(a)}$	$0.53 \pm 0.1^{(a)}$
Asteroseismology	$0.59 \pm 0.01^{(b)}$	$0.61 \pm 0.02^{(c)}$	$0.57 \pm 0.02^{(d)}$	$0.69 \pm 0.04^{(e)}$
By comparing $\overline {\Delta \Pi }$ with $\Delta \Pi _{\ell }^{\rm a}$ (this work)	$0.575 \pm 0.005$	$0.615 \pm 0.015$	$0.595 \pm 0.015$	$0.70 \pm 0.02$
By comparing $\overline {\Delta \Pi }$ with $\overline {\Delta \Pi _{{k}}}$ (this work)	$0.555 \pm 0.005$	$0.575 \pm 0.015$	$0.565 \pm 0.015$	$0.64 \pm 0.02$
By means of $\chi ^2$ period fittings (this work)	$0.56 \pm 0.01$	$0.60 \pm 0.01$	$0.55 \pm 0.01$	$0.64 \pm 0.01$

References: ^(a) Dreizler & Heber (1998); ^(b) KB94; ^(c) Reed et al. (2000);
^(d) Kawaler et al. (2004); ^(e) O'Brien (2000).