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between 2003 and 2006 we have been carrying out a systematic search for longitudinal magnetic fields in chemically peculiar stars whose magnetic field has never been studied before ( e.g. , * ? ? ?
* ; * ? ? ?
the goal of this study was to statistically enlarge our data sample by including all southern stars for which the position in the h - r diagram is known from accurate hipparcos @xcite parallaxes ( @xmath3 ) and from photometric data in the geneva and strmgren systems , used to determine their effective temperatures .
the longitudinal magnetic field determinations have been obtained with fors1 at the vlt , which is a multi - mode instrument equipped with polarisation analyzing optics comprising super - achromatic half - wave and quarter - wave phase retarder plates , and a wollaston prism with a beam divergence of 22@xmath4 in standard resolution mode .
we used the grism600b and the grism1200 g to cover several hydrogen balmer lines at a spectral resolution of r@xmath52000 for the grism600b and r@xmath54000 for the grism1200 g .
a few stars with detected strong longitudinal magnetic fields have been successively scheduled for the observations with high - resolution spectrographs .
in the course of such a study we had discovered a very slowly rotating extreme magnetic ap star , hd154708 ( = cd@xmath6576753 ) , which has the second highest value of a field modulus ever measured in ap and bp stars . up to now , the number of magnetic stars with a measured mean magnetic field modulus using magnetically resolved lines was 49 . in this paper
we present our most recent discovery of two additional strongly magnetic slowly rotating chemically peculiar stars , the sreucr star hd92499 ( = cd@xmath6426407 ) and the bp sicr star hd157751 ( = cd@xmath63312069 ) , for which the zeeman pattern is resolved for several spectral lines .
the observations of hd92499 and hd157751 have been carried out with the echelle spectrograph feros at the 2.2 m telescope at la silla .
the spectrum of hd92499 has been obtained on may 13 , 2006 , and the spectrum of hd157751 on june 14 , 2006 .
the s / n ratio of the spectra is about 200300 per pixel in the one - dimensional spectrum around 6150 .
the wavelength coverage is @xmath7 , and the nominal resolving power is 48000 . to our knowledge , these observations are the first carried out for both stars at high spectral resolution .
similar to the recently discovered cool low mass ap star hd154708 , hd92499 and hd157751 have never been studied in detail and the simbad database offers merely a few references . as mentioned above , the only studies of both stars have been conducted with the purpose of longitudinal magnetic field determination with fors1 at the vlt .
the longitudinal magnetic fields have been determined from the measurements of the circular polarisation of opposite sign induced in the wings of each balmer line by the zeeman effect ( see * ? ? ?
* ; * ? ? ?
* for more details ) .
our measurements using all hydrogen balmer lines from h@xmath8 to the balmer jump at a spectral resolution of r@xmath52000 revealed for both stars kilogauss longitudinal magnetic fields @xmath9 of the order of @xmath61.2 kg for hd92499 and 4.0 kg for hd157751 @xcite . in figs .
[ fig : iv92499 ] and [ fig : iv157751 ] we present both , the stokes i and v / i spectra of these stars in the vicinity of the h@xmath8 line .     line
. , scaledwidth=45.0% ]     line . , scaledwidth=45.0% ]    the visual inspection of high resolution feros spectra disclosed that in both stars spectral lines are strongly affected by the magnetic field .
several spectral line profiles observed in hd92499 are fully resolved whereas for hd157751 the magnetic splitting is less noticeable due to non - negligible rotational doppler effect . from the observed magnetically split lines we determined the mean magnetic field modulus ( that is the average over the visible stellar hemisphere of the modulus of the magnetic vector , weighted by the local line intensity ) which is related to the wavelength separation of the zeeman components through the relation @xmath10 where @xmath11 is the central wavelength of the line ( in a zeeman triplet this corresponds to the position of the @xmath12-component ) , @xmath13 is the wavelength separation between the centroids of the @xmath14-components and @xmath15 is the effective land factor . to calculate @xmath13 ,
the wavelengths of the centres of gravity of the split doublet and triplet components have been determined by fitting a gaussian simultaneously to each of them ( if the lines are not fully split ) or by direct integration of the whole component profile ( if the splitting is large ) .
when the lines were blended , a multiple fit of three or four gaussians has been performed .
this procedure is well - established over the years and has been used in previous studies of magnetic stars with resolved magnetically split lines ( e.g. , * ? ? ?
* ) . in this manner we obtained the mean magnetic field modulus
@xmath16 = @xmath17 kg for hd92499 and @xmath16 = @xmath18 kg for hd157751 .
the accuracy of our measurements essentially depends on the magnitude of the zeeman splitting , presence of blends and the complexity of the profiles .    strongly magnetic stars with measured mean magnetic fields from magnetically resolved lines are of special interest because they provide the best opportunity to study the effect of the magnetic field on the stellar atmospheres .
very frequently , abundances of many elements are non - uniformly distributed over the stellar surface and they are also vertically stratified . to get more insight in the peculiar nature of hd92499 and hd157751 , we tried to determine abundances of a few elements    to calculate the effective temperature @xmath19 and @xmath20 we used @xmath21 and geneva photometry listed in the general catalogue of photometric data @xcite .
further we made use of the software package templogg @xcite which offers the possibility to automatically correct for the reddening , to obtain @xmath22k , @xmath23 for hd92499 , and @xmath19= 11380k , @xmath20=4.44 for hd157751 .
the calculations of abundances for a few lines - mainly for fe and cr lines - were made with an evolved version of the width9 code @xcite .
this code takes into account the extra broadening induced by the magnetic field and gives a good first estimate which can later be refined by more detailed analyses using spectral synthesis .
iteratively , the dependence of the average fe abundance on excitation potentials of the measured lines was minimised to derive a more accurate value of @xmath19 .
the best fit to the observed spectrum of hd92499 has been achieved for @xmath19 = @xmath24k , and @xmath20 = @xmath25 . for hd157751
, we obtained @xmath19 = @xmath26k , and @xmath20 = @xmath27 .
our estimates of the projected velocity @xmath28@xmath29 using several fe and cr lines with small land factors resulted in @xmath28@xmath29 = @xmath30kms@xmath2 for hd92499 and @xmath28@xmath29 = @xmath31kms@xmath2 for hd157751 .
atomic line data were taken from vald @xcite .
iteratively , the derived fe and cr abundances were then used to calculate a model with individual abundances using llmodels code @xcite .
atmospheric parameters and abundances of fe  ii and cr  ii model calculations are presented in table  [ tab : results ] .    employing the calculated model for hd92499 , we have determined abundances for a few rare earth elements using strong unblended lines .
europium , praseodymium and neodymium have been found strongly overabundant , as typical for most magnetic chemically peculiar stars .
the computed respective relative abundances are : @xmath32 , @xmath33 , @xmath34 , @xmath35 , @xmath36 with estimated uncertainties of 0.3 dex .
interestingly , we find the obvious imbalance of nd and pr abundances from different ions , also called as `` rare - earth - anomaly '' .
the abundances derived from the first and second ions differ by @xmath50.86  dex for nd and by @xmath51.18  dex for pr .
such an anomaly is usually explained by vertical abundance stratification of the elements in the stellar atmosphere and is known to be a typical feature of the abundance pattern of the rapidly oscillating ap ( roap ) stars @xcite .
it is generally accepted that the vertical abundance stratification is a consequence of atomic diffusion , i.e. radiative levitation and gravitational settling .    the observed feros spectra and line identification according to vald in various spectral regions containing magnetically split lines
are presented for both stars in figs .
[ fig : hd92499_6149_onlyobs][fig : hd157751_6130 - 6133_onlyobs ] .
.atmospheric parameters and abundances of fe  ii and cr  ii used for spectrum synthesis . [
cols="<,^,^,^",options="header " , ]     6147.7 and @xmath376149.2 .
, scaledwidth=45.0% ]    .
, scaledwidth=45.0% ]     and @xmath38 .
, scaledwidth=45.0% ]    .
, scaledwidth=45.0% ]
strong magnetic stars with low @xmath39 values , for which the magnetic splitting of the spectral lines is evident in high resolution spectra in unpolarised light , present an excellent opportunity to derive stronger constraints on the geometry of their magnetic fields from the observations of the variability of the magnetic field modulus and the longitudinal magnetic field over the rotation cycle
. further information about the magnetic field geometry can be obtained from the study of the field moments measured in the circular polarisation profiles and in line profiles observed in unpolarised light as it has been done by @xcite .
additional observations of these two stars are needed to allow to determine their rotational periods , the magnetic field geometry and to carry out a multi - element abundance analysis .
our preliminary abundance analysis of the feros spectra of hd92499 and hd157751 reveals the overabundance of cr in both stars , overabundance of fe in hd157751 and a nearly solar abundance of fe in hd92499 .
the discovery of ionisation anomalies of nd and pr in hd92499 indicates the abundance pattern typical of roap stars . because of the location of hd92499 in the same region of parameter space in which rapidly oscillating ap stars
have been detected , this star was searched photometrically for pulsations by @xcite with no detected oscillations .
however , we recall the recent paper by @xcite reporting the discovery of the presence of a single frequency of @xmath40mhz ( @xmath41min ) with very low amplitudes in the range @xmath42ms@xmath2 from a high - precision radial velocity study of hd154708 . no oscillations had previously been detected in this star in photometric studies .
we conclude that a time resolved high resolution spectroscopic study of radial velocity is called for to confirm or withdraw the presumption that hd92499 could be a low - amplitude roap star for which photometry would not detect oscillations .
this work was supported by the austrian science fund ( fwf - pp17890 ) .
hubrig , s. , north , p. , szeifert , t. , 2005a , in : `` astronomical polarimetry : current status and future directions '' , proceedings of the conference held 15 - 19 march , 2004 in waikoloa , hawaii , usa . edited by a.  adamson , c.  aspin , c.j .
davis , and t.  fujiyoshi , p.374
we report the discovery of resolved magnetically split lines in two chemically peculiar stars , the sreucr star hd92499 and the bp sicr star hd157751 . from feros spectra , we have measured a mean magnetic field modulus of 8.5 kg for hd92499 and a mean magnetic field modulus of 6.6 kg for hd157751 . 
 both stars have small projected rotational velocities : @xmath0 = 3.0  km@xmath1s@xmath2 and 8.5  km@xmath1s@xmath2 , respectively . 
 our preliminary abundance analysis reveals ionisation imbalance of rare earths in hd92499 indicating the abundance pattern typical of rapidly oscillating ap stars . 
 cr and fe are found strongly overabundant in hd157751 .    
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