Prof. Chris Koen
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Position:  Senior Professor 
Department:  Statistics and Population Studies Programme 
Faculty:  Faculty of Natural Sciences 
Qualifications:  PhD (Astronomy), PhD (Mathematical Statistics) 
Research publications in this repository  
ORCID iD  0000 0003 2291 2660 
Tel:  021 959 3258 
Fax:  021 959 2909 
Email:  ckoen@uwc.ac.za 
Recent Submissions

The analysis of indexed astronomical time series – XII. The statistics of oversampled Fourier spectra of noise plus a single sinusoid
(Oxford University Press, 2015)With few exceptions, theoretical studies of periodogram properties focus on pure noise time series. This paper considers the case in which the time series consists of noise together with a single sinusoid, observed at ... 
The analysis of indexed astronomical time series – XI. The statistics of oversampled white noise periodograms
(Oxford University Press, 2015)The distribution of the maxima of periodograms is considered in the case where the time series is made up of regularly sampled, uncorrelated Gaussians. It is pointed out that if there is no oversampling, then for large ... 
A search for pmode pulsations in white dwarf stars using the Berkeley Visible Imaging Tube detector
(Oxford University Press, 2014)We present highspeed photometry (resolution 0.1 s) obtained during the commissioning of the Berkely Visible Imaging Tube system on the Southern African Large Telescope (SALT). The observations were an attempt to search ... 
An O−C (and light travel time) method suitable for application to large photometric databases
(Oxford University Press, 2014)The standard method of studying period changes in variable stars is to study the timing residuals or O−C values of lightcurve maxima or minima. The advent of photometric surveys for variability, covering large parts of ... 
Multicolour time series photometry of the T Tauri star CVSO 30
(Oxford University Press, 2015)Five consecutive runs of at least five hours, and two shorter runs, of V(RI)C time series photometry of CVSO 30 are presented. No evidence could be seen for planetary transits, previously claimed in the literature for ... 
Multicolour time series photometry of the variable star 1SWASP J234401.81−212229.1
(Oxford University Press, 2014)1SWASP J234401.81212229.1 may be one of a handful of contact binaries comprising two Mdwarfs. Modelling of the available observations is complicated by the fact that the radiation of the eclipsing system is dominated ... 
Multicolour time series photometry of four shortperiod weaklined T Tauri stars
(Oxford University Press, 2015)The paper describes continuous photometric monitoring of four premainsequence stars, probable members of young stellar associations. Measurements, covering at least four nights per star, were obtained by cycling through ... 
A detection threshold in the amplitude spectra calculated from Kepler data obtained during K2 mission.
(Oxford University Press, 2015)We present our analysis of simulated data in order to derive a detection threshold which can be used in the prewhitening process of amplitude spectra. In case of groundbased data of pulsating stars, this threshold is ... 
Multivariate comparisons of the period–lightcurve shape distributions of Cepheids in five galaxies
(Oxford University Press, 2007)A number of published tests suitable for the comparison of multivariate distributions are described. The results of a small power study, based on realistic Cepheid log period – Fourier coefficient data, are presented. ... 
Multicolour time series photometry of three periodic ultracool dwarfs
(Oxford University Press, 2013)Photometry in I, or contemporaneously in I and R, of the known variable ultracool dwarfs Kelu1 and 2MASS J11553952−3727350 is presented. The nature of the variability of Kelu1 appears to evolve on timescales of a day ... 
HE0230–4323: an unusual pulsating hot subdwarf star
(Oxford University Press, 2007)HE 0230−4323 is a known binary, consisting of a subdwarf star and a companion which is not observable in the optical. Photometric measurements reported in this paper have shown it to be both a reflectioneffect and a ... 
The Nyquist frequency for time series with slight deviations from regular spacing
(Oxford University Press, 2009)The paper is based on the notion that the Nyquist frequency νN is a symmetry point of the periodogram of a time series: the power spectrum at frequencies above νN is a mirror image of that below νN. Koen showed that the ... 
Fitting powerlaw distributions to data with measurement errors
(Oxford University Press, 2009)If X, which follows a powerlaw distribution, is observed subject to Gaussian measurement error e, thenX+e is distributed as the convolution of the powerlawand Gaussian distributions. Maximumlikelihood estimation of ... 
The analysis of indexed astronomical time series – X. Significance testing of O − C data
(Oxford University Press, 2006)It is assumed that O − C (‘observed minus calculated’) values of periodic variable stars are determined by three processes, namely measurement errors, random cycletocycle jitter in the period, and possibly longterm ... 
Estimation of the coherence time of stochastic oscillations from modest samples
(Oxford University Press, 2012)‘Quasiperiodic’ or ‘solarlike’ oscillations can be described by three parameters – a characteristic frequency, a coherence time (or ‘quality factor’) and the variance of the random driving process. This paper is concerned ... 
Photometry of the magnetic white dwarf SDSS 121209.31+013627.7
(Oxford University Press, 2006)The results of 27 h of time series photometry of SDSS 121209.31+013627.7 are presented. The binary period established from spectroscopy is confirmed and refined to 0.061 412 d (88.43 min). The photometric variations are ... 
The analysis of irregularly observed stochastic astronomical timeseries – I. Basics of linear stochastic differential equations
(Oxford University Press, 2005)The theory of loworder linear stochastic differential equations is reviewed. Solutions to these equations give the continuous time analogues of discrete time autoregressive timeseries. Explicit forms for the power ... 
Improved SAAO–2MASS photometry transformations
(Oxford University Press, 2007)Nearinfrared photometry of 599 stars is used to calculate transformations from the South African Astronomical Observatory (SAAO) JHK system to the TwoMicron AllSky Survey (2MASS) JHKS system. Both severalterm formal ... 
Detection of an increasing orbital period in the subdwarf B eclipsing system NSVS 14256825
(Oxford University Press, 2012)New timings of eclipses made in 2010 and 2011 are presented for the hot subdwarf B (sdB) eclipsing binary NSVS 14256825. Composed of an sdB star and a much cooler companion, with a period near 0.1104 days, this system ... 
An analysis of pulsation periods of long period
(Royal Statistical Society, 2007)Summary: We report the results of a period change analysis of time series observations for 378 pulsating variable stars. The null hypothesis of no trend in expected periods is tested for each of the stars. The tests are ...