Lutz-Kelker Bias Research Articles

Gaia calibrated UV luminous stars in LAMOST

We take advantage of Gaia Data Release 2 to present 275 and 1774 ultraviolet luminous stars in the far ultraviolet (FUV) and near ultraviolet (NUV), respectively. These stars exceed their expected values by 5σ with respect to over one million ultraviolet stars in the log g vs. Teff diagram. Galactic extinction is corrected with a 3D dust map. In order to limit the Lutz-Kelker bias to an insignificant level, we select stars with relative uncertainties in luminosity less than 40% and trigonometric parallaxes less than 20%. We cross-identified our sample with the catalogs of RR Lyr stars and possible white dwarf main-sequence binaries, and find they compose ∼62% and ∼16% of our sample in the FUV and NUV, respectively. This catalog provides a unique sample to study stellar activity, spectrally unresolved compact main-sequence binaries and variable stars.

Testing the Distance Scale of the Gaia TGAS Catalogue by the Kinematic Method

We have studied the simultaneous and separate solutions of the basic kinematic equations obtained using the stellar velocities calculated on the basis of data from the Gaia TGAS and RAVE5 catalogues. By comparing the values of Omega_0' found by separately analyzing only the line-of-sight velocities of stars and only their proper motions, we have determined the distance scale correction factor p to be close to unity, 0.97+/-0.04. Based on the proper motions of stars from the Gaia TGAS catalogue with relative trigonometric parallax errors less than 10% (they are at a mean distance of 226 pc), we have found the components of the group velocity vector for the sample stars relative to the Sun (U,V,W)o=(9.28,20.35,7.36)+/-(0.05,0.07,0.05) km s^{-1}, the angular velocity of Galactic rotation Omega0=27.24+/-0.30 km s^{-1} kpc^{-1}, and its first derivative Omega_0'=-3.77+/-0.06 km s^{-1} kpc^{-2}; here, the circular rotation velocity of the Sun around the Galactic center is V_0=218+/-6 km s^{-1} kpc (for the adopted distance R_0=8.0+/-0.2 kpc), while the Oort constants are A=15.07+/-0.25 km s^{-1} kpc^{-1} and B=-12.17+/-0.39 km s^{-1} kpc^{-1}, p=0.98+/-0.08. The kinematics of Gaia TGAS stars with parallax errors more than 10% has been studied by invoking the distances from a paper by Astraatmadja and Bailer-Jones that were corrected for the Lutz-Kelker bias. We show that the second derivative of the angular velocity of Galactic rotation Omega_0"=0.864+/-0.021 km s^{-1} kpc^{-3} is well determined from stars at a mean distance of 537 pc. On the whole, we have found that the distances of stars from the Gaia TGAS catalogue calculated using their trigonometric parallaxes do not require any additional correction factor.

PARALLAX RESULTS FROM URAT EPOCH DATA

ABSTRACT We present 1103 trigonometric parallaxes and proper motions from the United States Naval Observatory Robotic Astrometric Telescope (URAT) observations taken at the Naval Observatory Flagstaff Station (NOFS) over a three-year period from 2012 April to 2015 June covering the entire sky north of about decl. We selected two samples: previously suspected nearby stars from known photometric distances and stars showing a large, significant parallax signature in URAT epoch data without any prior selection criteria. All systems presented in this paper have an observed parallax ≥40 mas with no previous published trigonometric parallax. The formal errors on these weighted parallax solutions are mostly between 4 and 10 mas. This sample gives a significant (of the order of 50%) increase to the number of known systems having a trigonometric parallax to be within 25 pc of the Sun (without applying Lutz–Kelker bias corrections). A few of these are found to be within 10 pc. Many of these new nearby stars display a total proper motion of less than 200 mas yr−1. URAT parallax results have been verified against Hipparcos and Yale data for stars in common. The publication of all signifigant parallax observations from URAT data is in preparation for CDS.

High-precision timing of 42 millisecond pulsars with the European Pulsar Timing Array

We report on the high-precision timing of 42 radio millisecond pulsars (MSPs) observed by the European Pulsar Timing Array (EPTA). This EPTA Data Release 1.0 extends up to mid-2014 and baselines range from 7-18 years. It forms the basis for the stochastic gravitational-wave background, anisotropic background, and continuous-wave limits recently presented by the EPTA elsewhere. The Bayesian timing analysis performed with TempoNest yields the detection of several new parameters: seven parallaxes, nine proper motions and, in the case of six binary pulsars, an apparent change of the semi-major axis. We find the NE2001 Galactic electron density model to be a better match to our parallax distances (after correction from the Lutz-Kelker bias) than the M2 and M3 models by Schnitzeler (2012). However, we measure an average uncertainty of 80\% (fractional) for NE2001, three times larger than what is typically assumed in the literature. We revisit the transverse velocity distribution for a set of 19 isolated and 57 binary MSPs and find no statistical difference between these two populations. We detect Shapiro delay in the timing residuals of PSRs J1600$-$3053 and J1918$-$0642, implying pulsar and companion masses $m_p=1.22_{-0.35}^{+0.5} \text{M}_{\odot}$, $m_c = 0.21_{-0.04}^{+0.06} \text{M}_{\odot }$ and $m_p=1.25_{-0.4}^{+0.6} \text{M}_{\odot}$, $m_c = 0.23_{-0.05}^{+0.07} \text{M}_{\odot }$, respectively. Finally, we use the measurement of the orbital period derivative to set a stringent constraint on the distance to PSRs J1012$+$5307 and J1909$-$3744, and set limits on the longitude of ascending node through the search of the annual-orbital parallax for PSRs J1600$-$3053 and J1909$-$3744.

Why the distance of PSR J0218+4232 does not challenge pulsar emission theories

Recent very long baseline interferometry (VLBI) measurements of the astrometric parameters of the millisecond pulsar J0218+4232 by Du et al. have suggested that this pulsar is as distant as 6.3 kpc. At such a large distance, the large γ-ray flux observed from this pulsar would make it the most luminous γ-ray pulsar known. This luminosity would exceed what can be explained by the outer gap and slot-gap pulsar emission models, potentially placing important and otherwise elusive constraints on the pulsar emission mechanism. We show that the VLBI parallax measurement is dominated by the Lutz–Kelker bias. When this bias is corrected for, the most likely distance for this pulsar is | $3.15^{+0.85}_{-0.60}$ | kpc. This revised distance places the luminosity of PSR J0218+4232 into a range where it does not challenge any of the standard theories of the pulsar emission mechanism.

Calibration of RAVE distances to a large sample of Hipparcos stars

A magnitude-limited population of 18 808 Hipparcos stars is used to calibrate distances for 52 794 RAdial Velocity Experiment (RAVE) stars, including dwarfs, giants and pre-main-sequence stars. I give treatments for a number of types of bias affecting calculation, including bias from the non-linear relationship between the quantity of interest (e.g., distance or distance modulus) and the measured quantity (parallax or visual magnitude), the Lutz–Kelker bias and bias due to variation in density of the stellar population. The use of a magnitude bound minimizes the Malmquist and the Lutz–Kelker bias, and avoids measurement bias resulting from the greater accuracy of Hipparcos parallaxes for brighter stars. The calibration is applicable to stars in 2MASS when there is some way to determine stellar class with reasonable confidence. For RAVE this is possible for hot dwarfs and using log g. The accuracy of the calibration is tested against Hipparcos stars with better than 2 per cent parallax errors, and by comparison of the RAVE velocity distribution with that of Hipparcos, and is found to improve upon previous estimates of luminosity distance. An estimate of the local standard of rest from RAVE data, (U0, V0, W0) = (14.9 ± 1.7, 15.3 ± 0.4, 6.9 ± 0.1) km s−1, shows excellent agreement with the current best estimate from extended Hipparcos compilation. The RAVE velocity distribution confirms the alignment of stellar motions with spiral structure.

Corrections for the Lutz-Kelker bias for Galactic masers

Based on published data, we have collected information about Galactic maser sources with measured distances. In particular, 44 Galactic maser sources located in star-forming regions have trigonometric parallaxes, proper motions, and radial velocities. In addition, ten more radio sources with incomplete information are known, but their parallaxes have been measured with a high accuracy. For all 54 sources, we have calculated the corrections for the well-known Lutz-Kelker bias. Based on a sample of 44 sources, we have refined the parameters of the Galactic rotation curve. Thus, at R_0=8 kpc, the peculiar velocity components for the Sun are (Uo,Vo,Wo)=(7.5,17.6,8.4)+\-(1.2,1.2,1.2) km s^-1 and the angular velocity components are \omega_0=-28.7+\-0.5 km s^-1 kpc^-1, \omega'_0=+4.17+\-0.10 km s^-1 kpc^-2, and \omega''_0=-0.87+\-0.06 km s^-1 kpc^-3. The corresponding Oort constants are A=16.7+\-0.6 km s^-1 kpc^-1 and B=-12.0+\-1.0 km s^-1 kpc^-1; the circular rotation velocity of the solar neighborhood around the Galactic center is V_0 = 230+\-16 km s^-1. We have found that the corrections for the Lutz-Kelker bias affect the determination of the angular velocity \omega_0 most strongly; their effect on the remaining parameters is statistically insignificant. Within the model of a two-armed spiral pattern, we have determined the pattern pitch angle i=-6.5^\circ and the phase of the Sun in the spiral wave \chi_0=150^\circ.

ON PULSAR DISTANCE MEASUREMENTS AND THEIR UNCERTAINTIES

Accurate distances to pulsars can be used for a variety of studies of the Galaxy and its electron content. However, most distance measures to pulsars have been derived from the absorption (or lack thereof) of pulsar emission by Galactic H i gas, which typically implies that only upper or lower limits on the pulsar distance are available. We present a critical analysis of all measured H i distance limits to pulsars and other neutron stars, and translate these limits into actual distance estimates through a likelihood analysis that simultaneously corrects for statistical biases. We also apply this analysis to parallax measurements of pulsars in order to obtain accurate distance estimates and find that the parallax and H i distance measurements are biased in different ways, because of differences in the sampled populations. Parallax measurements typically underestimate a pulsar's distance because of the limited distance to which this technique works and the consequential strong effect of the Galactic pulsar distribution (i.e., the original Lutz–Kelker bias), in H i distance limits, however, the luminosity bias dominates the Lutz–Kelker effect, leading to overestimated distances because the bright pulsars on which this technique is applicable are more likely to be nearby given their brightness.

CHANDRAOBSERVATIONS OF THE OLD PULSAR PSR B1451–68

We present 35 ks Chandra ACIS observations of the 42 Myr old radio pulsar PSR B1451-68. A point source is detected 0.32" +/- 0.73" from the expected radio pulsar position. It has ~200 counts in the 0.3-8 keV energy range. We identify this point source as the X-ray counterpart of the radio pulsar. PSR B1451-68 is located close to a 2MASS point source, for which we derive 7% as the upper limit on the flux contribution to the measured pulsar X-ray flux. The pulsar spectrum can be described by either a power-law model with photon index Gamma=2.4 (+0.4/-0.3) and a unrealistically high absorbing column density N(H)= (2.5 (+1.2/-1.3)) * 10^(21) cm^-2, or by a combination of a kT=0.35 (+0.12/-0.07) keV blackbody and a Gamma = 1.4 +/- 0.5 power-law component for N(H)[DM]= 2.6 * 10^(20) cm^-2, estimated from the pulsar dispersion measure. At the parallactic, Lutz-Kelker bias corrected distance of 480 pc, the non-thermal X-ray luminosities in the 0.3-8 keV energy band are either Lx(nonth)= (11.3 +/- 1.7) * 10^(29) erg/s or Lx(nonth)= (5.9 (+4.9/-5.0)) * 10^(29) erg/s, respectively. This corresponds to non-thermal X-ray efficiencies of either eta(nonth)= Lx(nonth) / (dE/dt) ~ 0.005 or 0.003, respectively.

Lutz-Kelker bias in pulsar parallax measurements

Lutz & Kelker showed that parallax measurements are systematically overestimated because they do not properly account for the larger volume of space that is sampled at smaller parallax values. We apply their analysis to neutron stars, incorporating the bias introduced by the intrinsic radio luminosity function and a realistic Galactic population model for neutron stars. We estimate the bias for all published neutron star parallax measurements and find that measurements with less than ~95% certainty, are likely to be significantly biased. Through inspection of historic parallax measurements, we confirm the described effects in optical and radio measurements, as well as in distance estimates based on interstellar dispersion measures. The potential impact on future tests of relativistic gravity through pulsar timing and on X-ray--based estimates of neutron star radii is briefly discussed.

HIPPARCOS CALIBRATION OF THE TIP OF THE RED GIANT BRANCH

We have detected the tip of the Red Giant Branch (TRGB) in the solar neighborhood using near infrared photometry from the 2MASS and DIRBE catalogs, and revised Hipparcos parallaxes. We confirm that the revised Hipparcos parallaxes are superior to the original ones, and that this improvement is necessary to detect the TRGB. We find a tip absolute magnitude of M_K=-6.85+/-0.03, in agreement with that expected from previous tip measurements of the Large Magellanic Cloud, Small Magellanic Cloud, and Bulge. This represents the first geometric calibration of the TRGB and extends previous calibrations, based on metal-poor globular clusters, to solar metallicities. We attempted to use the TRGB to confirm the presence of the Lutz-Kelker bias, with inconclusive results. Attempts to detect the tip in the I-band also produced inconsistent results, due to a lack of precise, homogeneous photometry for these bright stars.

New absolute magnitude calibrations for detached binaries

AbstractLutz‐Kelker bias corrected absolute magnitude calibrations for the detached binary systems with main‐sequence components are presented. The absolute magnitudes of the calibrator stars were derived at intrinsic colours of Johnson‐Cousins and 2MASS (Two Micron All Sky Survey) photometric systems. As for the calibrator stars, 44 detached binaries were selected from the Hipparcos catalogue, which have relative observed parallax errors smaller than 15% (σπ/π ≤ 0.15). The calibration equations which provide the corrected absolute magnitude for optical and near‐infrared pass bands are valid for wide ranges of colours and absolute magnitudes: –0.18 < (B – V)0 < 0.91, –1.6 < MV < 5.5 and –0.15 < (J – H)0 < 0.50, –0.02 < (H – Ks)0 < 0.13, 0 < MJ < 4, respectively. The distances computed using the luminosity‐colours (LCs) relation with optical (BV) and near‐infrared (JHKs) observations were compared to the distances found from various other methods. The results show that new absolute magnitude calibrations of this study can be used as a convenient statistical tool to estimate the true distances of detached binaries out of Hipparcos' distance limit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Absolute Magnitudes of Be Stars Based on Hipparcos Parallaxes

The absolute visual magnitudes of 457 Be stars are determined from Hipparcos parallaxes, subsequently the mean absolute visual magnitudes of Be stars for luminosity classes III, IV and V are obtained. The new Mv calibration is compared with existing calibrations. It is found that Be stars are generally brighter than B stars of the corresponding spectral types, and that there is no direct correlation between absolute magnitudes and the stellar rotational velocity, these results are in agreement with some earlier results. A new and interesting result is that there is no direct correlation between near infrared excess and absolute visual magnitudes for Be stars. Moreover, possible biases, such as the Malmquist bias and the Lutz-Kelker bias, are also discussed.

A Bayesian Analysis of the Cepheid Distance Scale

We develop and describe a Bayesian statistical analysis to solve the surface brightness equations for Cepheid distances and stellar properties. Our analysis provides a mathematically rigorous and objective solution to the problem, including immunity from Lutz-Kelker bias. We discuss the choice of priors, show the construction of the likelihood distribution, and give sampling algorithms in a Markov chain Monte Carlo approach for efficiently and completely sampling the posterior probability distribution. Our analysis averages over the probabilities associated with several models rather than attempting to pick the best model from several possible models. Using a sample of 13 Cepheids we demonstrate the method. We discuss diagnostics of the analysis and the effects of the astrophysical choices going into the model. We show that we can objectively model the order of Fourier polynomial fits to the light and velocity data. By comparison with theoretical models of Bono et al. we find that EU Tau and SZ Tau are overtone pulsators, most likely without convective overshoot. The period-radius and period-luminosity relations we obtain are shown to be compatible with those in the recent literature. Specifically, we find log(R) = (0.693 ± 0.037) [log(P) − 1.2] (2.042 ± 0.047) and Mv = −(2.690 ± 0.169) [log(P) − 1.2] − (4.699 ± 0.216).

Is there really a Lutz--Kelker bias? Reconsidering calibration with trigonometric parallaxes

In the recent literature there are indications of some confusion regarding the Lutz–Kelker bias: whether or not it exists, and if so, what it is and when it should be corrected. Here we carefully reexamine Lutz & Kelker's original work to understand what they actually did, and then look at their later papers and some other works on the subject. There is, properly speaking, no universal Lutz–Kelker bias of individual parallaxes. There is a bias for stars that are members of samples which is different from, but often has the same form as and is given the name of, the Lutz–Kelker bias. The overall bias for samples selected according to relative parallax error is sometimes given the name of Lutz–Kelker; in fact it is, or is very nearly the same as, that discussed by Trumpler and Weaver. The Lutz–Kelker corrections can, under certain conditions, be used to counter that bias. The Lutz–Kelker correction applied for an isolated star (independent of sample properties) is an incomplete refinement of the estimate of absolute magnitude calculated directly from the parallax, not a correction for bias. We reconsider the more general problem of calibration using only trigonometric parallaxes, and examine some of the maximum likelihood methods proposed for its solution. Of these, several are based on linear approximation and are therefore of limited validity. Three exact methods are all based on essentially the same form of the likelihood but are implemented in different ways. One of these is statistically flawed, as was originally pointed out by Jung. We test the other two using synthetic samples, compare their performance, and discuss their application. We also apply one (grid method) to the Feast–Catchpole high weight sample of Hipparcos Cepheid parallaxes as a test. The grid method is to be preferred over the approximate ones because it does not have a limited range of validity, should not require any a posteriori correction, and provides a more complete picture of the uncertainties, in the form of a contour diagram of log(likelihood).

Bias Properties of Extragalactic Distance Indicators. XI. Methods to Correct for Observational Selection Bias for RR Lyrae Absolute Magnitudes from Trigonometric Parallaxes Expected from the [ITAL]Full-Sky Astrometric Mapping Explorer[/ITAL] Satellite

A short history is given of the development of the correction for observation selection bias inherent in the calibration of absolute magnitudes using trigonometric parallaxes. The developments have been due to Eddington, Jeffreys, Trumpler & Weaver, Wallerstein, Ljunggren & Oja, West, Lutz & Kelker, after whom the bias is named, Turon Lacarrieu & Creze, Hanson, Smith, and many others. As a tutorial to gain an intuitive understanding of several complicated trigonometric bias problems, we study a toy bias model of a parallax catalog that incorporates assumed parallax measuring errors of various severities. The two effects of bias errors on the derived absolute magnitudes are (1) the Lutz-Kelker correction itself, which depends on the relative parallax error δπ/π and the spatial distribution, and (2) a Malmquist-like incompleteness correction of opposite sign due to various apparent magnitude cutoffs as they are progressively imposed on the catalog. We calculate the bias properties using simulations involving 3 × 106 stars of fixed absolute magnitude using Mv = +0.6 to imitate RR Lyrae variables in the mean. These stars are spread over a spherical volume bounded by a radius 50,000 pc with different spatial density distributions. The bias is demonstrated by first using a fixed rms parallax uncertainty per star of 50 μas and then using a variable rms accuracy that ranges from 50 μas at apparent magnitude V = 9 to 500 μas at V = 15 according to the specifications for the Full-Sky Astrometric Mapping Explorer (FAME) satellite to be launched in 2004. The effects of imposing magnitude limits and limits on the observer's error, δπ/π, are displayed. We contrast the method of calculating mean absolute magnitude directly from the parallaxes where bias corrections are mandatory, with an inverse method using maximum likelihood that is free of the Lutz-Kelker bias, although a Malmquist bias is present. Simulations show the power of the inverse method. Nevertheless, we recommend reduction of the data using both methods. Each must give the same answer if each is freed from systematic error. Although the maximum likelihood method will, in theory, eliminate many of the bias problems of the direct method, nevertheless the bias corrections required by the direct method can be determined empirically via Spaenhauer diagrams immediately from the data, as discussed in the earlier papers of this series. Any correlation of the absolute (trigonometric) magnitudes with the (trigonometric) distances is the bias. We discuss the level of accuracy that can be expected in a calibration of RR Lyrae absolute magnitudes from the FAME data over the metallicity range of [Fe/H] from 0 to -2, given the known frequency of the local RR Lyrae stars closer than 1.5 kpc. Of course, use will also be made of the entire FAME database for the RR Lyrae stars over the complete range of distances that can be used to empirically determine the random and systematic errors from the FAME parallax catalog, using correlations of derived absolute magnitude with distance and position in the sky. These bias corrections are expected to be much more complicated than only a function of apparent magnitude because of various restrictions due to orbital constraints on the spacecraft.

The Spectroscopic and Astrometric Parallaxes of Three Dwarf Novae: The Nature of the Secondary Stars of U Geminorum, SS Aurigae, and SS Cygni

We compare the parallaxes of three dwarf novae obtained using the Fine Guidance Sensors on the Hubble Space Telescope with their ground-based infrared spectroscopic parallaxes to examine the nature of their secondary stars. We have used new optical spectroscopy of MK spectral-type standards to rework the spectroscopic parallaxes of the reference frames used by Harrison et al. for each dwarf nova. The recalibrated parallaxes for U Gem, SS Aur, and SS Cyg have then been corrected for the Lutz-Kelker bias. We present the proper motions of the three objects, finding space motions consistent with an old disk population. New moderate-resolution infrared spectroscopy of the three dwarf novae, covering the spectral range 2.18 to 2.30 μm, have been used to determine the spectral types of the secondary stars. We find an M1 spectral type for the secondary star of SS Aur, an M4 for U Gem, and a K4 for SS Cyg, in agreement with previous determinations. Using these spectral types and the astrometric parallaxes, we have deconvolved the spectral energy distributions for all three dwarf novae. Within the errors, the JHK photometry of SS Aur is consistent with a normal M1 V supplying its entire infrared luminosity. However, the infrared luminosities of both U Gem and SS Cyg are too high to be supplied by normal main-sequence stars of their observed spectral types. We investigate models for the nonstellar components of all three dwarf novae by subtracting various cool star and white dwarf spectra from the observed data. For U Gem and SS Aur we find that power-law residual spectra can be extracted for the nonstellar components of these two systems. When this analysis is performed for SS Cyg, however, we find that two additional luminous components remain.

On the determination of absolute magnitude zero-points from Hipparcos parallaxes

The period–luminosity (PL) relation zero-point determination from Hipparcos trigonometric parallaxes of Cepheids is contentious. It is shown that the method used by Feast &38; Catchpole is equivalent to a standard minimization of sum of squares, and that it is free of Lutz–Kelker bias. The same technique is applied to RR Lyrae and field HB star data, the PL relation being replaced by a luminosity–metallicity relation.

The Lutz--Kelker bias in trigonometric parallaxes

The theoretical prediction that trigonometric parallaxes suffer from a statistical effect, has become topical again now that the results of the Hipparcos satellite have become available. This statistical effect, the so-called Lutz-Kelker bias, causes measured parallaxes to be too large. This has the implication that inferred distances, and hence inferred luminosities are too small. Published analytic calculations of the Lutz-Kelker bias indicate that the inferred luminosity of an object is, on average, 30% too small when the error in the parallax is only 17.5%. Yet, this bias has never been determined empirically. In this paper we investigate whether there is such a bias by comparing the best Hipparcos parallaxes which ground-based measurements. We find that there is indeed a large bias affecting parallaxes, with an average and scatter comparable to predictions. We propose a simple method to correct for the LK bias, and apply it successfully to a sub-sample of our stars. We then analyze the sample of 26 `best' Cepheids used by Feast & Catchpole (1997) to derive the zero-point of the fundamental mode pulsators and leads to a distance modulus to the Large Magellanic Cloud - based on Cepheid parallaxes- of 18.56 +/- 0.08, consistent with previous estimates.

The Lutz-Kelker bias in trigonometric parallaxes

The theoretical prediction that trigonometric parallaxes suffer from a statistical effect, has become topical again now that the results of the Hipparcos satellite have become available. This statistical effect, the so-called Lutz-Kelker bias, causes measured parallaxes to be too large. This has the implication that inferred distances, and hence inferred luminosities are too small. Published analytic calculations of the Lutz-Kelker bias indicate that the inferred luminosity of an object is, on average, 30% too small when the error in the parallax is only 17.5%. Yet, this bias has never been determined empirically. In this paper we investigate whether there is such a bias by comparing the best Hipparcos parallaxes which ground-based measurements. We find that there is indeed a large bias affecting parallaxes, with an average and scatter comparable to predictions. We propose a simple method to correct for the LK bias, and apply it successfully to a sub-sample of our stars. We then analyze the sample of 26 ‘best’ Cepheids used by Feast & Catchpole (1997) to derive the zero-point of the PeriodLuminosity relation. The final result is based on the 20 fundamental mode pulsators and leads to a distance modulus to the Large Magellanic Cloud - based on Cepheid parallaxes- of 18.56 ± 0.08, consistent with previous estimates.